THE OCCURRENCE OF COPPER-PORPHYRIN COMPLEXES IN SOIL HUMIC ACIDS

Electron paramagnetic resonance (e.p.r.) spectroscopy has been used to show that humic acids, which were isolated from a range of soils and from which most of the iron was removed by boiling with hydrochloric acid, contain copper in the form of porphyrin complexes. The amounts of porphyrin-bound copper in the acid-boiled humic acids were estimated to be ≤ 20 mg/kg (equivalent to 0.05-0.9 mg/kg in the soils). The acid-boiled humic acids from cultivated mineral soils showed no great capacity for further uptake of copper as copper porphyrin when treated with CuSO₄, most of this additional copper being held by groups not involving N-donor groups. In contrast, copper treatment of the acid-boiled humic acid from a raised bog peat produced a large increase in copper-porphyrin signal strength, indicating that in the peat there are considerable amounts of porphyrin by which added copper could be complexed. It is suggested that porphyrin groups may be involved in the fixation of copper by organic matter in soils.